VIENNA (AP) - The U.S. is obligated by a United Nations convention to prosecute Bush administration lawyers who allegedly drafted policies that approved the use of harsh interrogation tactics against terrorism suspects, the U.N.'s top anti-torture envoy said today.

Earlier this week, President Barack Obama left the door open to prosecuting Bush administration officials who devised the legal authority for gruesome terror-suspect interrogations. He had previously absolved CIA officers from prosecution.

Manfred Nowak, who serves as a U.N. special rapporteur in Geneva, said Washington is obligated under the U.N. Convention against Torture to prosecute U.S. Justice Department officials who wrote memos that defined torture in the narrowest way in order to justify and legitimize it, and who assured CIA officials that their use of questionable tactics was legal.

"That's exactly what I call complicity or participation" to torture as defined by the convention, Nowak said at a news conference. "At that time, every reasonable person would know that waterboarding, for instance, is torture."

Nowak, an Austrian law professor, said it was up to U.S. courts and prosecutors to prove that the memos were written with the intention to incite torture.

Nowak also said any probe of questionable CIA interrogation tactics must be independent and have thorough investigative powers.

"It can be a congressional investigation commission, a special investigator, but it must be independent and with thorough investigative powers," Nowak said.

On Thursday, Obama's press secretary suggested Obama does not care for an independent panel.

Last week, the Obama administration released secret CIA memos detailing interrogation tactics sanctioned under Bush.

The memos authorized keeping detainees naked, in painful standing positions and in cold cells for long periods of time. Other techniques included depriving them of solid food and slapping them. Sleep deprivation, prolonged shackling and threats to a detainee's family also were used.

Nowak said Saturday that Obama's decision not to prosecute CIA operatives who used questionable interrogation practices violates the same U.N. convention. But at that point he did not specifically address the issue of how the convention would apply to those who drafted the interrogation policy and gave the CIA the legal go-ahead.